1. Field of the Invention
The present invention relates to a perovskite manganese oxide thin film. More specifically, the present invention relates to a perovskite manganese oxide thin film the electrical, magnetic or optical properties of which are switched in response to a stimulus such as temperature, electrical field, magnetic field or light exposure.
2. Background of the Related Art
There has been concern in recent years that semiconductor devices may be facing the limits of the scaling law, which has been a guiding principle of performance advances in the field. In this context, materials are being developed that will make possible new operating principles in order to weather the crisis when the transistor limit is reached. For example, in the field of spintronics, which exploits the spin degrees of freedom of electrons, there has been development aimed at high-density non-volatile memories capable of high-speed operation at the same level as DRAM (dynamic random access memory).
There has also been progress in research into materials having strongly correlated electron systems that cannot be described in terms of band theory, which is the cornerstone of semiconductor device design. Substances have been discovered that exhibit very large and rapid changes in physical properties caused by phase changes in the electron system. In strongly correlated electron system materials, a variety of electron phases with a variety of orders formed by spins, charges and orbitals are possible because the phase state of the electron system is affected not only by the spin degrees of the freedom but also by the degrees of freedom of the electron orbitals. Typical examples of strongly correlated electron system materials are the perovskite manganese oxides, in which a first order phase transition produces a charge-ordered phase by alignment of 3d electrons of manganese (Mn) and an orbital-ordered phase by alignment of the electron orbitals.
In a charge-ordered phase or orbital-ordered phase, electrical resistance increases because the carrier is localized, and the electron phase becomes an insulator phase. The magnetic behavior of this electron phase is that of an antiferromagnetic phase due to the double exchange interactions. The electron states of the charge-ordered phase and orbital-ordered phase should often be regarded as semiconductor states. This is because although the carrier is localized in the charge-ordered phase and orbital-ordered phase, the electrical resistance is lower than that of a so-called band insulator. In accordance with convention, however, the electron phases of the charge-ordered phase and orbital-ordered phase are here called insulator phases. Conversely, when the behavior is metallic with low resistance, the electron phase is a ferromagnetic phase because the spins are aligned. The term “metallic phase” is defined in various ways, but here a metallic phase is one in which “the temperature derivative of resistivity is positively signed”. Expressed in this way, the aforementioned insulator phase can be re-defined as one in which “the temperature derivative of resistivity is negative”.
A variety of switching phenomena have reportedly been observed in bulk single-crystal materials made of substances capable of assuming either the aforementioned charge-ordered phase or orbital-ordered phase, or a phase that combines both a charge-ordered phase and an orbital-ordered phase (charge- and orbital-ordered phase), see Japanese Patent Application Publication Nos. H08-133894, H10-255481, and H10-261291. These switching phenomena occur in response to applied stimuli, namely, temperature changes around the transition point, application of a magnetic or electric field, or light exposure. These switching phenomena are typically observed as very large changes in electrical resistance and antiferromagnetic-ferromagnetic phase transitions. For example, resistance changes by orders of magnitude in response to application of a magnetic field are a well-known phenomenon called colossal magnetoresistance.
To achieve an electronic device, magnetic device or optical device or any kind of device using these effects, the switching phenomena must be manifested when the perovskite manganese oxide has been formed as a thin film. As in the case of an ordinary semiconductor device, a defect-free single-crystal thin film is necessary in order to achieve high-performance switching properties and uniform properties. There has therefore been much research using laser ablation methods, which allow the preparation of high-quality thin films of perovskite manganese oxides. Due to advances in film-fabrication technology, it is now possible to prepare perovskite manganese oxide thin films while controlling the atomic layers one by one by monitoring the intensity oscillation of the RHEED (reflection high-energy electron diffraction).
SrTiO3 (lattice constant 0.3905 nm), LSAT ((LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7, lattice constant 0.387 nm) and other cubic perovskite single crystals are often selected as substrates for preparing perovskite manganese oxide thin films, and a (100) orientation is often used as the substrate plane orientation. The reasons for selecting such substrates have to do with lattice constant mismatch between the substrate and the perovskite manganese oxide. However, the problem has been that even if a single-crystal thin film of a perovskite manganese oxide is prepared by deposition on such a (100) oriented substrate, the switching phenomena are not manifested in the resulting (100) oriented perovskite manganese oxide single-crystal thin film. This is because the in-plane lattice of the formed thin film is fixed to the in-plane lattice of the substrate, and the first order phase transition to a charge-ordered phase or orbital-ordered phase requires a kind of lattice deformation called Jahn-Teller deformation, which is suppressed by in-plane fourfold symmetry of the substrate.
On the other hand, Japanese Patent Application Publication No. 2005-213078 discloses a perovskite oxide thin film formed using a substrate with a (110) orientation. According to this disclosure, the formed thin film allows shear deformation of the crystal lattice during switching when the in-plane fourfold symmetry of the (110) oriented substrate is broken. That is, in a thin film formed in accordance with Japanese Patent Application Publication No, 2005-213078 the crystal lattice is oriented parallel to the substrate plane, while the charge-ordered plane or orbital-ordered plane is non-parallel to the substrate plane. As a result, first order phase transitions involving deformation of the crystal lattice are possible even with a single crystal thin film the in-plane crystal lattice of which is fixed to the in-plane lattice of the substrate. Thus, according to Patent Document 4, a transition or in other words a switching phenomenon at high temperatures equivalent to those obtained with the bulk single crystal can be achieved by using a (110) oriented substrate.
As discussed above, the operating principle of these switching phenomena is a phase transition of an electron phase, namely a charge-ordered phase or orbital-ordered phase. To achieve a highly practical electronic device using a perovskite manganese oxide, the transition temperature to the charge-ordered phase or orbital-ordered phase must be brought to near the normal operating temperature of the device, or more specifically to an absolute temperature of 300 K or more or in other words room temperature or above. However, in the examples disclosed by prior art, a major barrier to practical use has been that the switching phenomena are manifested at room temperature or below both with the bulk and thin-film form of the oxide.